The present invention relates to an optical input detection apparatus adapted to detected whether a light beam emitted from a light emitting device to a photodetector has been interrupted.
There has been known such a device as shown in FIG. 1, comprising a detection panel 3 constituted by arranging a plurality of pairs of light emitting devices 1 and photodetectors 2 opposedly in a predetermined spaced relationship and adapted to detect the location of an object 4 on the panel by successively activating the pair of light emitting devices.
FIG. 2 schematically illustrates the constitution of the processing circuit for the received light signals including a pair of light emitting devices 1 and the photodetector 2 on the panel. In FIG. 2, the light output from the light emitting devices 1, which are driven and controlled by the microprocessor 5 through the light emitting device multiplexer is received by a corresponding pair of photodetectors 2 and converted into electrical signals. The signals are then supplied to the amplifying circuit 6 via the photodetector multiplexer and amplified to output signals of a specified level. The amplified signals are input to the comparator 7 where they are compared to the reference level and the compared output is then supplied to the microprocessor 5. More specifically, as shown in FIG. 3(A), if the light emitting device 1 is driven in a specified cycle, the received output as shown in FIG. 3(B) via the paired photodetectors 2 and the amplifying circuit 6 is input ot the comparator 7. However, if the light beam is interrupted by the object 4, no light is received. The comparator 7 is adapted to compare the output from the amplifier 6 with the reference level V.sub.f and provide "low" signals when the output exceeds the reference level and "high" signals when the output is less than the reference levels as shown in FIG. 3(C). The output from the comparator is taken in a constant timing while the light emitting device 1 is being driven and then output as shown in FIG. 3(D) and a decision is made that there is no object present in the case of a "low" signal and that there is an object present in the case of a "high" signal.
The operation to detect the presence of an object in the apparatus as shown in FIG. 3 may cause an erroneous detection whent the light source, namely an external flood light is very intense.
FIG. 4 illustrates an operation in which erroneous detection may be caused. In FIG. 4(B), when light is not interrupted by an object, the input to the comparator 7 corresponds to the amount of received light obtained by adding the output light from the light emitting device 1 and the external flood light and far exceeds the reference level V.sub.f [indicated by the dotted line in FIG. 4(B)] to be compared. Consequently, the output from the comparator 7 will be a "low" signal and may be detected as a sign of no object being present. However, when the light beam from the light emitting device 1 is interrupted by an object, the input corresponding to the external flood light is supplied to the comparator 7 and when the value of the input is higher than the reference level, the output from the comparator 7 is "low" which may cause the presence of an object to be overloocked and a state of absence to be actually registered.
Accordingly an object of the present invention is to provide an optical input apparatus adapted to reliably detect the presence of an object without being adversely affected by an external flood light.